The present invention relates to an ultrasonic motor employed in vehicles. More particularly, the present invention pertains to an ultrasonic motor that lowers electromagnetic noise.
FIG. 8 illustrates a typical ultrasonic motor 50. The motor 50 has a metal base 54 and a metal cover 58. The base 54 and the cover 58 form a motor housing. The base 54 is secured, for example, to a vehicle body frame. The motor housing houses a stator 52 made of iron and a rotor 53 made of aluminum. The rotor 53 is pressed against the stator 52. A rotary shaft 55 is supported by the base 54. A ball bearing 56 is located between the shaft 55 and the base 54 to allow the shaft 55 to rotate relative to the base 54. The rotor 53 is secured to the shaft 55 to integrally rotate with the shaft 55.
The stator 52 includes a disk-shaped diaphragm 45, a stationary plate 46 fixed to the inner bottom surface of the diaphragm 45 and a ring-shaped piezoelectric element 51 secured to the outer bottom surface of the diaphragm 45. The stationary plate 46 is secured to the base 54 by screws. The diaphragm 45 has radially extending teeth 45a along its the circumference. The distal end 45b of each tooth 45a contacts the bottom surface of the rotor 53.
The piezoelectric element 51 is polarized and has an A-phase region 51a, a B-phase region 51b and a vibration detecting region 51c as shown in FIG. 9. Each of the regions 51a, 51b, 51c is connected to a lead wire 57a, 57b, 57c by an electrode, respectively. Further, the regions 51a-51c are connected to a common grounding lead wire 57d. As illustrated in FIG. 8, the lead wires 57a-57d are connected to a connector 59 located outside of the cover 58.
As illustrated in FIG. 9, the connector 59 is connected to an electronic control unit (ECU) 60 by a shielded line 61. The ECU 60 is located far from the ultrasonic motor 50. The shielded line 61 includes an A-phase power supply wire 61a, a B-phase power supply wire 61b, a feedback signal wire 61c and a grounding wire 61d. The ECU 60 applies high-frequency voltage to the A-phase region 51a through the A-phase power supply wire 61a, the connector 59 and the lead wire 57a. The ECU 60 also applies high-frequency voltage to the B-phase region 51b through the B-phase power supply wire 61b, the connector 59 and the lead wire 57b. There is a ninety-degree phase difference between the voltage sent to the A-phase region 51a and the voltage sent to the B-phase region 51b. The high frequency voltages vibrate the A-phase region 51a and the B-phase region 51b. The vibration vibrates the distal ends 45b of the teeth 45a with respect to the stator 52. The vibration of the distal ends 45b generates a progressive wave. The progressive wave rotates the rotor 53, which is pressed against the distal ends 45b. The rotation of the rotor 53 is transmitted to the rotary shaft 55.
The feedback signal wire 61c is connected to the vibration detecting region 51c by the connector 59 and the lead wire 57c. The region 51c generates voltage in accordance with vibration of the A-phase region and the B-phase region 51b and outputs the generated voltage to the ECU 60. The ECU 60 feedback controls the high frequency voltage applied to the A-phase region 51a and the B-phase region 51b based on this voltage. The grounding wire 61d is connected to the grounding lead wire 57b by the connector 59. The regions 51a-51c of the piezoelectric element 51 are grounded to the vehicle body frame through the ECU 60.
As illustrated in FIG. 8, the stationary plate 46 of the stator 52 is fastened to the base 54. The rotor 53 is pressed against the diaphragm 45 of the stator 52 and is coupled to the rotary shaft 55. The shaft 55 is supported by the bearing 56. That is, the stator 52 is directly and electrically connected the base 54 and is indirectly and electrically connected to the base 54 via the rotor 53, the rotary shaft 55 and the bearing 56.
Therefore, the regions 51a, 51b, 51c of the piezoelectric element 51 are grounded not only through the ECU 60 but also through the base 54. In other words, the ultrasonic motor 50 is directly grounded to the vehicle body frame. The impedance of the grounding wire 61d between the ultrasonic motor 50 and the ECU 60 is higher than the impedance of the body frame. Therefore, when the ECU 60 applies high frequency voltage to the motor 50, grounded current does not flow through the grounding wire 61d but flows to the ECU 60 through the vehicle body frame. The motor 50, the shielded line 61d and the body frame form a closed loop circuit. The closed loop circuit functions as a loop antenna, which generates electromagnetic noise. The farther from the motor 50 the ECU 60 is located, the larger the area surrounded by the closed loop circuit becomes. A larger area surrounded by the closed loop circuit increases the electromagnetic noise. The electromagnetic noise generates radio noise, which interferes with the sound of the car radio. The electromagnetic noise also adversely affects other communications devices.